This invention relates generally to circuits for driving audio acoustic energy devices; more particularly, the invention relates to a circuit which limits the acoustic energy presented to a telephone headset connected to a commercially-available telephone receiver.
A feature of the invention is that it uses very little current from the telephone battery for performing the function of limiting the acoustical energy presented to the telephone headset. In commercial telephone systems the level of acoustical energy delivered to the user""s ear is dependent on many variables, including the quality of the interconnecting electrical lines, the quality of the many electrical junctions and wiring connections between the sender and receiver, the quality of the amplification circuits along the way, and the existence of random or repeatable noise sources and sources of acoustic interference. In order to protect the ear of the user, it is desirable to provide a circuit for limiting the maximum acoustic energy which can be delivered to the ear. The present invention is concerned with this type of protection, afforded to users of telephone headsets having an independent acoustic transducer as an earpiece, and which is connectable to an ordinary, commercially-available telephone receiver in substitution for the normal handset associated with such a receiver.
A low voltage compression circuit serving as a feedback loop in a signal transmission path between a telephone receiver and a telephone headset earphone. The circuit is connected as a feedback loop in the operational amplifier circuit which amplifies the received acoustic energy from the telephone receiver; as the operational amplifier output signal increases beyond a predetermined output level, a portion of the operational amplifier circuit output signal is directed to a semiconductor switching circuit to increasingly turn on a shunt signal to the amplifier negative feedback input terminal, thereby increasing the negative feedback (decreasing the operational amplifier gain) and reducing the operational amplifier output signal.
It is a principal object of the present invention to provide a variable gain circuit in a telephone operational amplifier, which operates under very low voltage and current constraints, to thereby conserve energy drain from the telephone system battery and greatly extend battery life.
It is another object of the invention to provide a variable gain circuit in a telephone headset signal path, for limiting the acoustical energy presented to the headset.
Other objects and advantages of the invention will become apparent from the following specification, and with reference to the drawings and appended claims.